1. Field of the Invention
The field of the present invention relates generally to implanted devices for tissue stimulation, monitoring, and other therapeutic or diagnostic functions, and specifically to implantable devices for the stimulation of cardiac tissue, for example pacemakers or implantable cardioverter-defibrillators (ICDs). More specifically, it pertains to such devices utilizing wireless energy transfer, for example through ultrasonic means.
2. Description of the Background Art
Conventional cardiac pacemaker and defibrillator systems comprise Implantable Pulse Generators (IPGs) constructed with a single hermetically-sealed enclosure that contains the electronic control circuitry and battery components. The mass of an IPG for a conventional cardiac pacemaker is primarily dependent upon the volume and weight of the enclosed battery that powers the electronic control elements and stimulation energy of the IPG.
In order to have an IPG of acceptable size and weight for implantation in patients, the batteries on average do not last as long as the patients' lifetime. Cardiac pacemaker and defibrillator battery longevity ranges from about 3-9 years (Hauser et al., “Feasibility and Initial Results of an Internet-Based Pacemaker and ICD Pulse Generator and Lead Registry,” Pacing and Clinical Electrophysiology (PACE) 2001; 24:82-87). Therefore, many patients undergo multiple surgical procedures to replace the IPG.
For ease of insertion of pacing leads into the vasculature, the IPG is normally located subcutaneously in the pectoral region of the chest. This location is surgically accessible and near the typically used entry sites for the vascular leads. There is latitude with regards to the placement of the IPG enclosure. However, because it is connected to leads, the location and surgical process must consider lead insertion into a vascular access. Most often, the IPG is placed in a shallow subcutaneous pocket located in the upper left aspect of the chest, several centimeters below the clavicle. Venous access is then accomplished via the subclavian, cephalic, or axillary veins. This location generally allows the patient full range of movement with minimal discomfort. For example, see Hauser et al., 2001.
An ultrasound based Wireless Cardiac Stimulation (WiCS™) system has been disclosed in currently pending applications by the applicant (e.g., U.S. patent application Ser. No. 11/315,023). This system employs ultrasonic energy transfer from a subcutaneously implantable controller-transmitter device (C-T), which is directed towards one or more receiver-stimulator (R-S) devices implanted at desired sites in the heart, for example in the left ventricle. Ultrasonic transducers and circuitry in the R-S convert the transmitted ultrasonic energy into an electrical signal capable of stimulating the cardiac tissue. The WiCS™ system, C-T, and R-S are described in co-pending U.S. patent applications Nos. (Publication Number) 20060136004, 20060136005, 20070027508, 20070055184, 20070078490 and 20070060961 and Ser. No. 11/752,775, which are herein incorporated by reference in their entirety.
In the disclosed WiCS™ system, the C-T contains ultrasound elements as well as housing for the electronic control circuitry and battery. The C-T transmits ultrasound through the chest wall to the location of the R-S which may be placed in the left ventricle of the heart or any other heart location. In one preferred embodiment of the system, the transmitting aperture is located with a direct line of transmission between the C-T and the R-S—the direct line avoiding both bone and lung tissue to minimize transmission losses. Thus, it is desirable to place the transmitting aperture anteriorly over the rib cage of the left chest, and further desirable to align as much of the aperture of the transmitter as possible with the intercostal space between two ribs. It is still further desirable to tightly affix the transmitting aperture in the preferred location so that other factors, such as patient movement, do not disturb its position.
However, it can be observed that a significant portion of the general population, including potential recipients of the WiCS™ system, do not have significant thickness of muscle or fat tissue in the left anterior chest in the area of the 4th, 5th, and 6th ribs, which are preferred locations for such a device. Therefore, to improve patient comfort and to provide the largest usable aperture, it would be desirable to optimize the volume of devices implanted in this area, particularly by reducing the thickness of the device. To reduce the possible movement or dislodgement of the device, it would also be desirable to optimize its weight, particularly by reducing the mass of the device. The present embodiments address these challenges, as well as providing further advantages, as disclosed herein. Alternatively, other implant sites that have an unobstructed acoustic path between the C-T and R-S, for example, pericardial placement or subcostal placement, would be more practical if the C-T could remain undisturbed when replacing the battery.